I know I have a long time to prepare for this, but I still wonder how its all going to play out:

I've been step mom to michael since he was 8 months old. His bio mom literally up and left one day and never came back. She has not seen him since he was only 2 months old. He is growing and developing with me as his mother, so naturally he is calling and knowing me as mom.

What age would you tell him that I am not his birth mother? and how would you go about telling him?

I don't think there is an age to sit him down and tell him. My brother was raised the same way. His birth mom dropped him off and he was rasied by my mom. He knows her as mom and has never questioned that. His case was different in the fact that he would see her and talk to her but those times were so rare that it was more confusing I think than anything. I think when my mom said anything was when he asked about it. Again his case was different in knowing his birth mom to some degree. He did have some issues with it later on. When he was mad at her or something he would scream that she wasn't his mom and that he was adopted. He thought he was adopted for some time and was angry about that. I think as long as you let him know the circumstances (you know explain it only to the degree he asks...don't go overboard) he'll gradually integrate it into his being. HTH!

Give more**Expect LessThere is no such thing as bad weather. Only bad clothing.

I would honestly say 8-12. My reasons is that at that age, its something they could get used to quickly and its not in the teens where it will bite ya in the butt, every time they get mad at you. I'm sure it might be said here and there. But if this is something you don't want to hide and want him to be comfortable with growing up I'd say then. Old enough to understand, but not too old to resent you from holding it from him either. what to say, ummm, maybe something along the lines of, that she was scared of the responcibility and didn't think she could give him the best life possible, and wanted to do the best thing for him that she knew how.
I don't really know, you don't want him to really hate her, and you don't want him to ever think he wasn't wanted by someone, especially his bio mom. That could really hurt his little heart. *hugs* this is a hard one.

When he starts asking where babies come from, IMO; or at least when he asks if he grew in your tummy.

You explain that he didn't grow in your tummy, that you have been his mommy since he was this big (use hands to show the size he was when you came into his life, or tell him the age) and that your heart opened up a special place to love him this much (big big wide arms) and that place never ever goes away.

As far as explaining where the mommy that grew him in her tummy goes, I just don't know the story, and can't think of anyhting right now to explain that . You're Mommy, she's the birth mother.

You never said how old he is now. I could see how it would be hard, if it hasn't been spoken about for years. However, I would still think that the sooner the better so he doesn't feel like it was a dark secret that was hide from him. So when he starts asking about where babies come from, I would def. tell him then. I would give the young child version of how there are many ways children can come into a family. Talk about how a woman carries them and mentioned that he was carried in his biological mother's belly, but etc. If it can be simply another piece of info. about him which isn't really that big a deal, I think it would be easier to handle. But if it made into a big deal that is kind of a dark secret, I could see issues coming from it.